In the manufacture of certain types of integrated circuits (ICs), such as high density dynamic random access memories (DRAMs) or static random access memories (SRAMs), it has been a common practice to deposit selected metals such as tungsten, W, in the form of surface layers on the outer surfaces of integrated circuit wafers. These metal layers extend down into vertical passages such as vias or troughs within the semiconductor wafer to make electrical contact with active devices formed in the underlying semiconductor substrate. For example, a tungsten layer has been deposited by chemical vapor deposition (CVD) on the surface of a titanium or titanium nitride underlayer and into openings (vias). These openings were previously etched through a dielectric underlayer of SiO.sub.2 and down to a silicon substrate where the IC active devices have been previously formed. The formation of these IC structures using tungsten plugs is described in more detail, for example, in copending application Ser. No. 07/734,908 of Fernando Gonzeles et al entitled "Self-Aligned Contact Device and Method For Making High Density Electrical Connections Through Semiconductor Memory Cells", filed Jul. 24, 1991, assigned to the present assignee and incorporated herein by reference.
Once electrical contact is made through the tungsten plugs to the active devices within the silicon substrate, it becomes necessary to chemically and mechanically polish the tungsten metal residue from the outer surface of the dielectric substrate (the titanium or titanium nitride underlayer) before proceeding further in the wafer fabrication process. This step is taken in order to reduce the resistivity of the electrical interconnection being made and to thereby improve device switching speed. The above chemical-mechanical polishing (CMP) process will typically use a combination of wet chemical etchant and a motor driven polishing pad to remove the outer tungsten layer down to the tungsten/titanium or titanium nitride interface. This process may also be used to remove other metals such as titanium nitride or titanium to an underlying oxide interface or to another equivalent underlayer.
In the past, in order to determine when the metal (e.g. tungsten)/underlayer (e.g. titanium nitride) interface was reached during the CMP process, the silicon wafer was often physically removed from the CMP machine and then inspected under a microscope to determine if the polished metal layer had been completely removed. Using other prior art layer monitoring techniques, certain contact monitoring methods have been employed which require some physical attachment to either the wafer or the CMP machine or both. Both of these prior methods are very labor intensive, and they have traditionally exhibited a low throughput.
Another, non-contact method for detecting when the metal/underlayer interface has been reached employs laser interferometry techniques. Such techniques require the use of expensive laser beam generation equipment and processing systems and also require both pre-CMP and post-CMP measurements which are also labor intensive. Accordingly, it is the elimination of the disadvantages of the above prior art contact and non-contact metal/underlayer interface monitoring systems and the solutions to the above problems to which the present invention is directed.